1. Field of the Invention
The present invention relates to a belt transport apparatus, an image forming apparatus, and an image forming system. More specifically, the present invention relates to a belt transport apparatus, an image forming apparatus, and an image forming system which are adapted to control a belt surface speed in a drive control of an intermediate transfer driving motor (which will be called a motor) for driving an intermediate transfer belt (which will be called a belt) using a sensor (linear scale sensor) to detect the surface speed of the belt and a sensor (encoder) to detect a speed of the intermediate transfer driving roller (which will be called a driving roller).
2. Description of the Related Art
In an image forming apparatus which controls a belt surface speed using the sensor (linear scale sensor) to detect the surface speed of the belt and the sensor (encoder) to detect the rotational speed of the driving roller, it is known that, when a toner image primarily transferred from a photoconductive drum onto the belt is transferred to a recording sheet, a secondary transfer roller is pressed against the belt and the recording sheet is interposed between the secondary transfer roller and the belt so that the toner image on the belt is transferred to the recording sheet.
In a case of a full-color image forming apparatus, toner images of multiple colors (which are typically the four colors: black, cyan, magenta and yellow) from corresponding photoconductive drums of the respective colors are sequentially superimposed on the belt, and it is important to maintain (control) the belt surface speed at a fixed level in order to prevent degradation of image quality, such as color deviation or banding (periodic density/color fluctuations in the sub-scanning direction of solid image).
However, in order to reduce the color deviation, it is desired that the sensor (linear scale sensor) that detects the belt surface speed measures a belt surface speed in an area where the toner images are superimposed. If the linear scale sensor is disposed in the primary transfer portion, the toner may be accumulated on a belt surface speed detection portion of the sensor depending on the structure of the sensor, and there is a possibility that the sensor is unable to detect accurate belt surface speed due to the accumulated toner.
In this respect, Japanese Laid-Open Patent Publication No. 2004-220006 discloses that, when the linear scale sensor is difficult to output a normal scale reading due to belt scale staining or the like, alternative control is performed in which a control using the linear scale sensor is switched to a control using the encoder.
Japanese Laid-Open Patent Publication No. 2005-092763 discloses that a belt surface speed detection result and a driving roller rotational speed detection result are selectively used as a control signal, and not only for a duration in which the belt surface speed detection result is found abnormal, but also throughout a duration from a driving roller stop state to a steady driving state, the control based on the driving roller rotational speed detection result is continuously performed.
Japanese Laid-Open Patent Publication No. 2009-222112 discloses a method of measuring the belt surface speed on the belt scale to control the belt surface speed by using double loops with the linear scale sensor and the driving shaft encoder, in which the belt surface speed is controlled using only the driving shaft encoder without using the belt scale sensor depending on the mode.
Furthermore, in a mode of an image forming apparatus in which five photoconductive drums are provided and only the photoconductive drum disposed in the most upstream position in the belt transport direction of the image forming apparatus is in contact with the belt, there may be a case in which the sensor to detect the belt surface speed cannot be used due to the internal structural layout of the apparatus. In such a case, the control of the belt surface speed is performed using only the encoder.
It is also known that when the normal reading of the belt scale sensor cannot be obtained, or in the above-described case, the alternative control is performed in which the control using the linear scale sensor is switched to the control using the encoder.
However, if the control of the belt surface speed is performed by using only the encoder as in the related art, for example, when the intermediate transfer belt driving roller is in an expanded state due to a rise of the atmospheric temperature, the belt surface speed may fluctuate according to the temperature change, and there is a problem that the accuracy of the belt surface speed control using only the encoder may be degraded.